No matter who you date, you are either going to break up or be with them forever. It's not an easy thought. If you aren't wanting to marry him in particular, then you have to realize you are going to break up at some point, there is no reason to stay with someone you aren't happy with. If you are happy with him and just aren't ready for marriage, then whatever. Enjoy your time dating and having fun, let him know you aren't ready but still want to be with him. I'm sure he'll respect and like that better than the notion that every time he talks about it you pull away.